3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as an evolved NodeB (eNodeB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the RBS is a typical example of a physical implementation of an eNodeB. More generally, a UE may be referred to as a wireless device.
FIG. 1 illustrates a radio access network in an LTE system. An eNodeB 101a serves a UE 103 located within the RBS's geographical area of service or cell 105a. The eNodeB 101a is directly connected to the core network. The eNodeB 101a is also connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. 
LTE Coordinated Multipoint (CoMP) is a facility that is being developed for LTE Advanced. LTE CoMP transmission and reception refers to a wide range of techniques that enable dynamic coordination or transmission and reception with multiple geographically separated eNodeBs. Its aim is to enhance the overall system performance, utilize the resources more effectively, and improve the end user service quality. The techniques used for CoMP are very different for the uplink (UL) and downlink (DL). This results from the fact that the eNodeBs are in a network connected to other eNodeBs, whereas the handsets or UEs are individual wireless devices.
One fundamental property of DL CoMP, that is expected to be relevant also for the evolution of LTE in Rel-12, is the possibility to transmit different signals and/or channels from different geographical locations or Transmission Points (TP) to the same UE. Correspondingly, in UL CoMP signals transmitted by a certain UE may be received and decoded at different reception point(s) (RP) than those associated to DL transmissions.
One of the principles guiding the design of the LTE system is transparency of the network to the UE. In other words, the UE is able to demodulate and decode its intended channels without specific knowledge of scheduling assignments for other UEs or network deployments. Furthermore, UEs are not aware of the geographical location or TP from which each signal is transmitted or of the location of the RP(s). In the following, TP will sometimes be used to refer to a TP or a RP or a combined TP/RP.
In order to allow transparency of the TPs to the UEs, UE specific Demodulation Reference Signals (DMRS) are employed to assist transmission of DL channels, including data channels called Physical Downlink Shared Channels (PDSCH), and control channels called Enhanced Physical Downlink Control Channels (ePDCCH).
Antenna Ports Quasi Co-Location (QCL)
In order to allow the UEs implementation to reach a reasonable compromise between complexity and performance, a number of assumptions have been specified related to the so called antenna ports QCL. QCL assumptions are specified in LTE and in certain cases signaled or configured by the network to the UEs. Such assumptions allow the UEs to jointly estimate certain long term channel properties for selected Reference Signal (RS) antenna ports. At the same time, joint estimation of certain long term channel properties is prohibited for other RS antenna ports. Even though QCL assumptions respect the NW transparency principle described above, they implicitly enable UEs to receive signals from different TPs on different ports or resources.
An example is ePDCCH, which carries Downlink Control Information (DCI) messages that may be transmitted from different TPs. In an ePDCCH region, i.e, the region dedicated for ePDCCH transmissions, the ePDCCH resources are partitioned in Physical Resource Block (PRB) sets, where each DCI message spans resources from a single PRB set. Even though ports within a PRB set are assumed to be quasi co-located, meaning that all DCI messages received on a given ePDCCH PRB set should be transmitted from the same TP, different PRB sets may be configured with different QCL assumptions, meaning that DCI messages received on different PRB sets may be transmitted from different TPs respectively.
Improved support for heterogeneous network operations is part of the ongoing specification of 3GPP LTE Release-12, and further improvements are discussed in the context of new features. In heterogeneous networks, a mixture of cells of differently sized coverage areas and with differently overlapping coverage areas is deployed. One example of such a deployment is the one where pico cells are deployed within the coverage area of a macro cell. In general, a macro node managing a macro cell is characterized by a larger transmission power and DL coverage as compared to a pico node managing a pico cell.
Control Signaling in LTE
Messages transmitted over a radio link to UEs can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, assignments of DL and UL resources and parameters for data communication, referred to as DL grants and UL grants, information regarding the quality of the radio links or other propagation parameters, higher layer signaling, and so on.
Examples of channels carrying DL control messages are the Physical Downlink Control Channels (PDCCH) in LTE and ePDCCH in LTE Advanced which for example carry scheduling information and power control messages, the Physical Hybrid automatic repeat request Indicator Channel (PHICH) which carries Acknowledgement (ACK) and Non-acknowledgement (NACK) in response to a previous UL transmission and the Physical Broadcast Channel (PBCH) which carries system information. Also the Primary and Secondary Synchronization Signals (PSS/SSS) can be seen as control signals. They have fixed locations, periodicity in time, and frequency so that UEs that initially access the network can find them and synchronize.
In the UL, control signaling comprises e.g. ACK/NACK sent in response to previous DL transmissions, feedback regarding the channel quality and recommended transport formats, scheduling requests, and buffer status reports. UL control channels are carried by Physical Uplink Control Channel (PUCCH) and are occasionally multiplexed on Physical Uplink Shared Channel (PUSCH).
In addition, Sounding Reference Signals (SRS) are transmitted in the UL in order to assist link adaptation. In case of Type 1 SRS, the SRS transmission is triggered by certain grants for data scheduling.
Starting from LTE Rel-11, a number of parameters affecting UL transmissions can be configured in a UE specific fashion. This includes, e.g., resources for PUCCH, base (scrambling) sequences for RS associated to PUSCH and PUCCH, and other parameters associated to UL RS generation, such as cyclic shift hopping initialization, and sequence group hopping enabling.
UL Power Control in LTE
UL power control in LTE is based on a hybrid algorithm including a closed loop part and an open loop part. The closed loop part includes incremental cumulative power adjustments commanded by the NW. The closed loop component may also be configured so that the command specify an absolute adjustment value to a higher-layer configured power level. The open loop part provides autonomous power adjustment at the UE based on partial compensation of the path loss estimated based on a given DL RS. The RS used for the serving cell path loss estimation is the Cell Specific RS (CRS). The UL Power Control (PC) process governs PUCCH, PUSCH and SRS, with configurable power offsets between such channels or signals.
During the latest concept developments, a number of improvements have been done in order to better utilize the channel and propagation properties of an LTE heterogeneous network deployment, sometimes also called Hetnet. These improvements enable better channel usage and can be an important component in enabling a deployment where the UL and DL originate from different locations. One example of such an improvement is the ePDCCH control channel, which is already standardized. The current standard works reasonably well with an ideal backhaul between TPs or nodes, i.e. a fast backhaul having a delay close to zero and a close to infinite bandwidth or bit-rate capacity. Information received by one RP can then without further delay be forwarded via the backhaul to the other RP. With a non-ideal backhaul such as a slow backhaul with a delay, the situation is not as good though, as control data forwarded to another RP will be received with a delay.
Another improvement that has been done is the decoupling of UL/DL. Conventionally, UL and DL transmissions are coupled, meaning that if a DL transmission is received from a certain TP, a corresponding UL transmission would be directed to the same TP. The decoupled UL/DL provides a part of the spectral efficiency, for example in a Hetnet deployment where the UL transmission may be directed to a close by low power Pico node and the DL reception may come from a high power multiple antenna Macro node. However, the spectral efficiency gain of the decoupled UL/DL cannot be fully realized in the current LTE standard, as e.g. the power control is designed with a coupled UL/DL in mind. The output power differences between Pico and Macro nodes can cause serious problems for code or spatially multiplexed channels. The channels can be heavily interfered due to the current design of power control were a coupled UL/DL is assumed. These kinds of problems may be present in any Hetnet deployment.